1. Field of the Invention
The present invention relates generally to medical electronic devices and, more particularly, relates to implantable devices useful for the correction of cardiac arrhythmias.
2. Description of the Prior Art
The problem of sudden patient deaths from ventricular tachyarrhythmias and particularly fibrillation has been addressed for some time. Nearly every intensive care unit and cardiac care unit within a major hospital has defibrillation equipment readily available. A characteristic of this equipment is that it is used externally and must be operated by trained medical personnel. A safe and effective implantable automatic defibrillator has been envisioned for some time. Such a device would be implanted in a patient having a high probability of sudden death from ventricular fibrillation and would operate automatically to cardiovert a patient's heart regardless of the patient's location of instant activity.
Mirowski et al, in U.S. Pat. No. 3,614,955, disclose an early implantable defibrillator. Employing an implantable pressure transducer to detect fibrillation. Specifically, whenever a pressure transducer located within the right ventricle ceases to record the normal pressure transitions attendant to pumping activity, a defibrillation pulse is generated and applied to the tissue. This approach has two basic difficulties. Although a great deal of work has been undertaken in the nearly ten years since the issue of the patent, effective chronically implantable pressure transducers are difficult to fabricate and are expensive to produce. The second problem with the device taught by Mirowski et al relates to the fact that the apparatus does not attempt to apply a cardioverting signal until such time as the patient's heart has actually entered into ventricular fibrillation.
A later and more sophisticated implantable defibrillator is taught by Rizk in U.S. Pat. No. 4,114,628. This device senses both the mechanical activity of the patient's heart and the electrical activity. As has become known, merely sensing electrical activity in terms of a normal ECG is insufficient to detect the onset of fibrillation because, in many patients, the right ventricle may appear to have a normal ECG signal but, in effect, not be pumping an adequate supply of blood. Therefore, Rizk utilizes a comparison circuit for comparing the mechanical activity with the electrical activity of heart. Such a comparison is used to determine whether or not the patient's heart is indeed properly responding to the electrical stimulus present. As with the approach of Mirowski, et al., this technique essentially awaits right ventricular fibrillation before a stimulation pulse is applied. As has been proven many times in actual experimentation, the greater the delay in applying the defibrillator pulse after the onset of ventricular fibrillation, the larger the energy required in the pulse to accomplish defibrillation. In addition, it is reasonable to expect that the longer defibrillation is delayed, the greater the trauma inflicted upon the patient. Furthermore, the size and cost of the implantable device is directly related to the amount of energy required to effect defibrillation, and it is desirable to reduce the energy required to both reduce patient trauma and to decrease the size and cost of the implantable device.
Electrodes for successful defibrillation after some delay may be seen in U.S. Pat. No. 4,030,509 issued to Heilman et al. As can be seen, these electrodes are prepared to handle relatively large energy stimulation pulses. Again, such large energy pulses are required if substantial delay is encountered between the onset of the initial ventricular tachycardia and the application of the cardioverting pulse.
A more recent implantable defibrillator is disclosed in U.S. Pat. No. 4,184,493 issued to Langer et al. The reference teaches detection of the onset of ventricular fibrillation by sophisticated processing of an intracardiac ECG signal. As discussed above, this technique may be very effective in many cases, however, it is known that for some period of time ventricular fibrillation may occur before it is detectable within the ECG signal. The effect of this delay is again the requirement that a larger energy is used for the defibrillating stimulation pulse.
One can see from these references and other available in the field deemed no more pertinent than these cited that the difficulty associated with the implantable defibrillator involves the use of the proper sensor or sensor combination offering a high reliability prediction of the onset of fibrillation. This prediction offers low energy and low patient trauma cardioversion. Indeed, the ultimate goal is prevention of ventricular fibrillation by early intervention.